It is known that outputs of an air-fuel ratio sensor are different from one another when hydrogen exists in a gas to be detected. Patent Document 1 discloses that the detection of switching from rich air-fuel ratio to lean air-fuel ratio is delayed in case where the hydrogen exists.
Patent Document 2 discloses that the output of the air-fuel ratio sensor tends to be richer under the influence of a rich gas containing hydrogen.    Patent Document 1: Japanese Unexamined Patent Publication No. 11-247687    Patent Document 2: Japanese Unexamined Patent Publication No. 2006-291893    Patent Document 3: Japanese Unexamined Patent Publication No. 8-49585    Patent Document 4: Japanese Unexamined Patent Publication No. 2006-152845    Patent Document 5: Japanese Unexamined Patent Publication No. 2003-120383